Isolation and identification of Bacillus vallismortis HSB-2 and its biocontrol potential against apple replant disease

• B. vallismortis HSB-2 was isolated from the rhizosphere of healthy apple trees. • HSB-2 inhibited mycelial growth and spore germination of Fusarium . • VOCs produced by strain HSB-2 have plant growth-promoting properties. • B. vallismortis HSB-2 reduces the disease severity and incidence of Fusarium . • B. vallismortis HSB-2 showed strong biocontrol ability on apple seedlings. To avoid the shortcomings of chemical fungicides currently used to control apple replant disease, in this study, HSB-2 was isolated from the rhizosphere soil of healthy apple trees, and in vitro experiments showed that it had an antagonistic effect on a variety of plant fungal pathogens, especially Fusarium oxysporum , Fusarium moniliforme , Fusarium proliferatum , and Fusarium solani . Its fermentation broth showed a significant inhibitory effect on the mycelial growth and spore germination of four Fusarium species. HSB-2 was identified as Bacillus vallismortis based on physiological and biochemical characteristics and molecular sequencing. HSB-2 produced diffusible and volatile organic compounds (VOCs) and showed good antifungal and growth-promoting properties. The pot experiment showed that HSB-2 had good colonization ability in the rhizosphere of Malus hupehensis Rehd . seedlings, and the disease index of seedlings inoculated with HSB-2 stabilized at 40, and the relative control effect reached 60% after 5 weeks. When 2.1 × 10 9 amount HSB-2 (at CFU·g −1 ) was added to the soil together with manure carrier (cow dung:straw = 3:1) (T2), it significantly increased the biomass of apple plant, the numbers of soil bacteria and actinomycetes, and activities of antioxidant enzymes, including superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) as well as soil enzymes, including urease, phosphatase, sucrose and CAT, and significantly reduced the content of phlorizin, benzoic acid, cinnamic acid, and p-hydroxybenzoic acid in soil. The results of T-RFLP and qPCR showed that T2 treatment significantly increased the carbon utilization level and Simpson index of bacteria, and decreased the abundance of four Fusarium species in soil, causing the bacterial and fungal community structures to differ significantly from those for other treatments. In conclusion, HSB-2 can protect apple plants from damage by four Fusarium species and phenolic acids, improve soil microbial community, increase soil enzyme activity, and promote plant growth. We thus propose the use of B. vallismortis HSB-2 as a biological control agent to control ARD.